1. Field of the Invention
The present invention relates in general to electronic circuitry for detecting rapid increases in line voltages and in particular to electronic circuitry for detecting rapid increases in line voltages despite a preceding decrease in line voltage. Still more particularly, the present invention relates to circuitry for detecting and eliminating rapid increases in line voltages applied to variable speed drives utilizing a phase controlled converter.
2. Description of the Prior Art
Variable speed drive systems are well known in the prior art. Such systems typically utilize three phase alternating current (AC) power which is rectified and utilized to create a direct current (DC) bus having a positive and negative rail. Phase controlled switching devices, such as silicon controlled rectifiers (SCR's), are generally utilized to vary the amount of power within the rectified signal by altering the phase angle at which the switching device operates. By selectively controlling the firing signal applied to the gates of these SCR's the amount of power passed to the DC bus may be accurately controlled.
In an alternating current (AC) drive systems inverter circuits are then typically utilized to create a variable frequency alternating current signal from the DC bus, which may be utilized to control a variable speed motor or other such device. Voltage controlled oscillators are generally utilized to generate inverter gate signals which may be utilized to selectively vary the frequency of the output signal, effectively controlling the frequency of operation of a motor or other load device. Of course, in a direct current (DC) drive system an inverter circuit will not be necessary.
As those skilled in the art will appreciate, the voltage level applied to the switching devices in such a circuit must be accurately controlled to avoid damage to the switching devices. Voltage regulation circuits are well known in the art and typically involve circuits which are utilized to "scrub" excess voltage when the line voltage exceeds a preselected level. A problem exists with such voltage regulation circuits in their inability to detect and respond to rapid increases in line voltage, particularly in situations in which the line voltage first "sags" and then rapidly increases.
It is therefore one object of the present invention to provide a protection circuit for use with variable speed drives utilizing phase controlled converters which can be utilized to detect and eliminate rapid increases in line voltages.